On Thu, Jul 08, 2010 at 09:30:23PM -0500, Louis Wasserman wrote:
> Consider
>> newtype Foo = Foo Int
>> lift :: (Int -> a) -> Foo -> a
> lift f (Foo x) = f x
>> Now, I'd expect this to compile with -O2 down to something like
>> lift f = f `cast` (Foo -> a)
>> but it doesn't.
>> It seems that GeneralizedNewtypeDeriving assumes that these two things *are*
> equivalent, and it just directly casts the class dictionary. The
> implication would be that that GeneralizedNewtypeDeriving gives more
> efficient instances than you could *possibly* get if you wrote them by hand,
> which is very sad.
It's true. For more, see this thread:
http://www.mail-archive.com/haskell-cafe@haskell.org/msg72300.html
Of course that thread is more about how sometimes GND gives you
*wrong* code. That's currently being worked on and will hopefully be
fixed at some point. But as you point out, it's also worth thinking
about the flip side: how to optimize away non-type-class-related
functions like your example.
-Brent